drm/ttm: roundup the shrink request to prevent skip huge pool

e.g. shrink reqeust is less than 512, the logic will skip huge pool

Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>